Geology and petrology of the Voisey's Bay intrusion: reaction of olivine with sulfide and silicate liquids

The Voisey's Bay Ni-Cu-Co sulfide deposit is located in Eastern Labrador, a nd transects the 1.85 Ga collisional boundary between interbanded garnet-si llimanite and quartzo-feldspathic paragneisses (Tasuiyak gneisses) of the P roterozoic Churchill Province to the west and granitic, intermediate and ma fic orthogneisses of the Archean Nain Province to the east. The deposit is associated with an upper troctolitic chamber (the Eastern Deeps chamber) wh ich is connected to a lower troctolitic chamber (the Reid Brook chamber) by a gabbroic/troctolitic feeder sheet approximately 1 km in vertical extent. Rock sequences that have been defined to date include Leuco-troctolite (LU T) in the Reid Brook chamber, Feeder Breccia (FB) in the Reid Brook Zone, F eeder Olivine Gabbro (FOG), Leopard Troctolite (LT) and Basal Breccia Seque nce (BBS) in the feeder sheet, and Olivine Gabbro (OG), Normal Troctolite ( NT) and Varied-textured Troctolite (VT) in the Eastern Deeps chamber. Miner alisation has been found in the feeder sheet, as massive stringers in gneis ses flanking the feeder, in the 'Ovoid', a 600 x 300 X 110 m(3) basin of ma ssive sulfide at surface, and as a zone of massive sulfide that has develop ed close to the entry of the feeder to the Eastern Deeps chamber. The miner alisation comprises pyrrhotite (both hexagonal and troilite in varying prop ortions), pentlandite, chalcopyrite, magnetite and minor cubanite. An impor tant component of all mineralized zones is a breccia of gneissic fragments in a troctolitic to noritic matrix referred to as BBS or FB. Considerable r eaction has occurred between the troctolite magma and the gneiss inclusions . Olivines in the LUT, OG and FOG have low Ni contents relative to their fo rsterite (Fo) contents. Olivines in the NT and VT have higher Ni contents b ut show a great scatter in both their Ni and Fo contents. The Ni content of olivine in the LT increases systematically with decreasing Fo content. All of these variations are explained in terms of fractional crystallisation o f magma, sulfide segregation, trapped Liquid shift and re-equilibration bet ween sulfide and olivine. Our current understanding of the development of t he deposit is that an initial pulse of magma rose to a lower chamber, fract ionated to form mafic-ultramafic cumulates and at the same time became sulf ide-saturated as a result of interacting with the enclosing, sulfide- and g raphite-bearing Tasiuyak gneisses. The magma lost much of its Ni and Cu as it fractionated olivine and segregated sulfides. A new wave of fresh magma entered the lower chamber, forcing the chalcophile-depleted magma, along wi th some of its contained sulfide, up the feeder to spread out in the upper chamber. Remnants of this chalcophile-depleted magma are preserved as olivi ne gabbro which forms upper layers in the upper chamber, and which in place s has frozen to the walls of the feeder. Sulfides that were swept up in thi s event were forced out into fractures in the feeder walls, are preserved w ithin thickened zones within the feeder, and were deposited as the magma fl ow decreased at the entry of the feeder to the upper chamber. Continued inf lux of the later, undepleted magma resulted in sulfides being picked up, tr ansported farther up the system, upgraded during transportation and then re deposited. (C) 1999 Elsevier Science B.V. All rights reserved.